The invention relates to a method for feeding chemicals into a fibre suspension in the short circulation of a paper machine. The invention also relates to a device for feeding chemicals into a fibre suspension in the short circulation of a paper machine.
Paper or board is a mixture comprising fibres, fines and different additives. Some of the paper components are mixed together already in the stock preparation department, but some of the chemicals are added to a finished stock mixture only in the short circulation a little before the stock is fed to a headbox.
From the standpoint of the properties of paper or board, it is very important that all raw material components are mixed so that they form a mixture that is as homogenous as possible when a web is formed. This requires an efficient system for feeding chemicals in the short circulation of the paper machine. It is particularly important that the chemicals are well mixed in the entire stock volume. For example, retention agents used for improving the retention of fines in a wire section must be mixed into the pulp as uniformly as possible to achieve maximal efficiency and to avoid variation in the properties of paper. Retention agents are usually fed before devices that generate shear stresses in the flow, such as a pump, a screen, or hydrocyclones. The chemicals are often fed into a pipe by means of a feed ring. A problem lies in causing the chemicals to be mixed uniformly in plug flow. It is also possible to use slot nozzles fitted crosswise inside a pipe. The problems with this arrangement include a high risk of contamination, and poor miscibility.
In general, it can be stated that a homogeneous mixture is produced most easily when the mixing volume is small and the turbulence enhancing mixing is sufficiently strong. In paper and board manufacturing processes, attempts have been made to make use of devices generating turbulence in the flow by dosing chemical components before a screen or a pump. When the rotor blades of a screen or a pump rotate quickly, very strong shear fields are created which generate turbulence. In practice, the thus generated shear fields may be even too strong, causing the break up of polymer chains, the activity of polymers as retention aids being thus weakened.
An object of the invention is to cause chemicals to be uniformly distributed in a fibre suspension, thus resulting in a homogeneous mixture. One further object of the invention is to carry out the feeding of chemicals so gently that polymer chains do not break up.